The field of this invention is communications systems, and more particularly, time division multiple access digital bus communications systems.
There are many forms of multiple access communications systems known in the art, including for example, the time division system disclosed in the incorporated reference. These systems basically provide an information bus for transferring digital message signals between remote terminals. In the prior art systems, there are a number of protocols utilized to accommodate the various terminals in the network and provide them with access to a communications bus. Such protocols include simple polling, priority request, contention, and cyclic time division with fixed slot allocations.
In the polling type systems, a central controller sequentially polls each of the remote terminals, offering each an opportunity to access the communications bus when available. In priority request systems, remote terminals ready to transmit a message make a request, and are granted access to the bus according to priorities established by a bus arbiter at a central controller. In contention systems, remote terminals may transmit messages at random times, monitor the signals on the bus, and subsequently re-transmit in the event two or more simultaneous transmissions are detected.
The polling and priority request approaches have been applied to time division systems in the prior art whereby a central terminal controls the bus access. However, such systems are typically characterized by a rigid formating of messages and an inflexible set of system constraints controlling the time periods at which the various remote terminals may gain access to the communication path. In addition, the various data rates at which the individual remote terminals may transmit message signals are hard-wired into the systems to define predetermined portions of the channel bandwidth which are allocated to each of the remote terminals.
The cyclic time division systems, including that disclosed in the incorporated reference, dedicate regularly occurring time slots in a repetitive framed sequence to specific users for their transmissions, and hence, may be referred to as a fixed slot allocation system. In the system described in the incorporated reference, the slot allocations may be tailored somewhat to specific user requirements through the procedure whereby remote terminals may request certain portions of the bus spectrum in predetermined access request time slots. In this system, a central controller then allocates families of message time slots to the requesting terminals in accordance with various parameters, such as predetermined priorities, time slot availabilities, and the like.
The system described in the incorporated reference overcomes these disadvantages to a large extent by permitting dynamic allocation of time slots in order to accommodate desired access time and data rate variations. However, in the referenced systems, all the control is achieved at a central terminal, which must control bus access and link formation for the entire system. Consequently, the level of complexity for that control terminal is relatively high. In addition, outage of that central terminal causes failure of the entire system.
Typically, in operating environments, a system is required to accommodate high duty cycle subscribers, low duty cycle subscribers, and combinations of both. The high duty cycle subscribers, such as inter-computer links, have generally been accommodated in the prior art by the cyclic time division systems wherein the various subscribers are assigned fixed slot allocations in the repetitive time framed time sequence on the bus. The low duty cycle, or bursty, subscribers, such as communications terminals, have been accommodated in the prior art by unslotted contention type systems, where the entire bus bandwidth is allocated to a single subscriber. Although these types of systems are specifically tailored for the particular types of subscribers, neither system type optimally accommodates both high and low duty cycle subscribers.
Accordingly, it is an object of the present invention to provide a time division multiple access communications system in which control of bus access and communication link formation is distributed over a plurality of terminals in the system.
It is another object to provide a time division multiple access communications system in which subscriber terminals contend for transmission time slots.
A further object is to provide a time division multiple access communications system which accommodates both high and low duty cycle subscribers by controlling slot allocations and contention slot transmissions.